How to fix rotational constraint 'jumping' in rotation problem
To get an object rotating at a constant rate you need to set two keyframes to define the speed of rotation then set the keyframe interpolation type to linear and the channels extrapolation mode to linear. This gives a constant rotation without speed variation.



To get another gear rotating in time with the main gear it is easy to setup a Transformation constraint. This constraint allows you to copy a rotation proportionally.



Here I have the source rotating -120 and the destination rotating 360. The key to having it rotate continuously is to tick the extrapolate check box.